The present invention relates to a thermo-sensitive actuator which generates driving force upon detecting a temperature change in an environment or a specific apparatus. The present invention also relates to an idle speed controller (hereinafter referred to as "ISC") which employs the thermo-sensitive actuator.
In some controlled systems, response conditions for control are desired to change in accordance with a temperature change in an environment or an apparatus. For example, there is an ISC mounted on an automobile. In this case, when the automobile is in an idling state, the intake air quantity is affected by the engine temperature.
Accordingly, temperature sensors are attached to necessary positions, and detected signals from the temperature sensors are collected to obtain control conditions. As specific temperature sensors, various types of sensor have already been proposed, such as those which employ wax, and those which employ a bimetal.
A specific example of changing response conditions of an apparatus in accordance with a temperature change will be explained below with regard to an automobile which is in an idling state. In the case of an automotive internal combustion engine, when the engine temperature is high, the bypass air quantity must be reduced, whereas, when the engine temperature is low, the bypass air quantity must be increased. In the case of a heater or similar device, when the temperature is low, it is necessary to increase the opening of the fuel valve to thereby increase the heat generation rate. In other words, a valve body for an ISC is required to move in such a manner that, when the temperature is low, the valve body moves with a large stroke, whereas, when the temperature is high, the valve body moves with a small stroke near the position for closing the valve to prevent runaway. FIG. 1 is a graph showing characteristics required for a rotary actuator for controlling the valve body of an ISC, in which temperature T .degree. C.! is plotted along the abscissa axis, and stroke deg! is plotted along the ordinate axis.
Accordingly, this type of control requires a plurality of temperature sensors or temperature-sensing actuators and also needs to constitute a control circuit taking into account temperature conditions. Consequently, the structure of the controller becomes complicated, and the reliability degrades. Further, as the number of constituent elements increases, the production cost rises.
With regard to control conditions, some valves as objects to be driven need to be closed (fully closed) irrespective of the temperature when the apparatus is not energized. A fuel valve used in a heater, for example, comes under this category. In this case, when the apparatus is in an inoperative state, the fuel valve must be fully closed to prevent fuel from flowing out.